The present invention relates, in general, to field emission devices, and, more particularly, to methods for operating field emission devices.
High voltage field emission devices (FED""s) are known in the art. A high voltage FED, is characterized by the application to an anode of the device of a potential greater than about 600 volts, typically more than 1000 volts. Illustrated in FIG. 1 is a partial, cross-sectional view of a prior an high voltage FED 100.
FED 100 includes a cathode plate 110, an anode plate 120 ), and a sealant 130, which are configured to provide a thin envelope. Cathode plate 110 is spaced apart from anode plate 120 to define an interspace region 111. Interspace region 111 is typically evacuated to a pressure of about 10% Torr. A separation distance, d, between anode plate 120 and cathode plate 110 is on the order of one millimeter.
Cathode plate 110 Includes a back plate 112, which is typically made from glass or silicon. Back plate 112 defines a proximate surface 155 and a distal surface 146. A cathode 113 is disposed on proximate surface 155. Cathode 113 is partially defined by a ballast resistor 114, which is a semiconductive layer. Cathode 113 also includes conductive portions, which are connected by ballast resistor 114. Cathode 113 is connected to an electron emitter 118 at one of the conductive portions, thereby operably coupling ballast resistor 111 to electron emitter 118. Cathode 113 supplies electrons to electron emitter 118. Ballast resistor 114 is useful for controlling the flow of electrons to electron emitter 119.
The distance between electron emitter 119 and distal surface 146 is greater than the distance between electron emitter 118 and proximate surface 155. That is, proximate surface 155 is proximately disposed with respect to electron emitter 118, and distal surface 196 is distally disposed with respect to electron emitter 110.
Cathode plate 110 further includes a dielectric layer 116, which is disposed on cathode 113 and defines an emitter well 117. Electron emitter 118 is disposed within emitter well 117. Dielectric layer 116 further defines a surface 140. A gate extraction electrode 119 is disposed upon a portion of surface 140 of dielectric layer 116.
Anode plate 120 is disposed to receive electrons emitted by election emitter 118. Anode plate 120 includes a transparent substrate 122, which is typically made from a glass. Transparent substrate 122 defines a proximate surface 153 and a distal surface 159, which are spaced apart from one another. Proximate surface 153 of transparent substrate 122 partially defines interspace region 111.
An anode 124 is disposed on a portion of proximate surface 153 of transparent substrate 122. Anode 124 is typically made from a transparent conductive material, such as indium tin oxide. A phosphor 126 is disposed upon anode 124. Phosphor 126 is cathodoluminescent and emits light upon activation by electrons.
As further illustrated in FIG. 1, a first voltage source 132 is connected to cathode 113, for applying a cathode voltage thereto; a second voltage source 134 is connected to gate extraction electrode 115, for applying a gate voltage thereto; and a third voltage source 136 is connected to anode 124, for applying an anode voltage thereto. During the operation of FED 100, the cathode voltage, the gate voltage, and the anode voltage are elected to cause and control an electron current 138 from electron emitter 118 and to attract the electrons toward phosphor 116. Electron current 138 can cause ionization of gaseous species that exist within interspace region 111, thereby creating a plurality of ionized species 142.
However, during the operation of prior art FED 100, several forces operate to undesirably change the electrical characteristics of FED 100. The undesirable changes are due at least in part to the presence of mobile electric charges within the components of FED 100.
For example, transparent substrate 122 contains a plurality of mobile charges 150. Because FED 100 is a high voltage device, the anode voltage is a high positive potential, which can be greater than 1000 volts. The high anode voltage causes positive charge within transparent substrate 122 to be repelled away from anode 124 and toward an edge 148 of transparent substrate 122. A build up of positive charge at edge 148 creates the risk of establishing a potential at proximate surface 153 which is sufficient to cause electric arcing over the surface of sealant 130 within interspace region 111. The risk of electric arcing is further exacerbated by the fact that the separation distance between anode plate 120 and cathode plate 110 is very small.
As a further example, back plate 112 has a plurality of mobile charges 144, which are also redistributed during the operation of FED 100. A force, which can cause this change in the distribution of charge, is the electrostatic force due to the accumulation of ionized species 142 at surface 140 of dielectric layer 116. Mobile charges 144 are repelled from proximate surface 155. A change in the charge distribution at proximate surface 155 causes a change in the conductivity of ballast resistor 114. Because ballast resistor 114 is a semiconductor the change in charge distribution at the underlying surface Because charges in the properties of the conductive channel of, ballast resistor 114. An uncontrolled change in the conductivity of ballast resistor 114 causes an undesirable change in the magnitude of electron current 138.
Accordingly, there exists a need for an improved field emission device, which overcomes at least these shortcomings of the prior art.